The present invention relates generally to fiber optics and optical switches. More particularly, the present invention relates to an optical switch tap for an all-optical or photonic switch.
Use of large-scale MEMS-based optical switches is the emerging choice for all-optical networks. As switch sizes grow from tens of ports to thousands of ports, the tap monitoring function becomes increasingly important. For optical cross-connect applications, an optical tap function provides signal performance monitoring including such parameters as power level, signal to noise ratio, wavelength, and bit error rate. The tap optics provide a method to selectively pick up any channel and monitor the selected channel performance.
A MEMS (micro-electro-mechanical system) device is a micro-sized mechanical structure having electrical circuitry. A MEMS can be fabricated using conventional integrated circuit (IC) fabrication methods. One type of MEMS device is a microscopic gimbaled mirror device. A gimbaled mirror device includes a mirror component, which is suspended off a substrate, and is able to pivot and to redirect light beams to varying angles within a three dimensional surface.
One type of optical switch includes arrays of electrically controlled MEMS mirror devices arranged in mirror arrays to selectively couple light beams from input fibers to output fibers. Such an optical switch is commonly referred to as an all-optical cross-connect switch or photonic switch. For each port of the switch, an input pivoting mirror is located in a path of a respective light beam being propagated by a respective input optical fiber. The input pivoting mirror is pivotable relative to the mirror substrate to alter an angle at which the light beam is reflected therefrom. The angle is controlled so that the light beam falls on a respective output pivoting mirror in line with a respective output optical fiber to which the light beam is to be switched. The output pivoting mirror then reflects the light beam and is pivoted so as to ensure that a light beam is propagated in a direction in which the output optical fiber extends, to ensure coupling of the light beam into the output optical fiber.
FIG. 1 shows an example of a prior art MEMS gimbaled mirror device used to redirect light beams in an optical switch. Light beams from fibers 1 located in input fiber array 2 are input to the optical switch and travel through input lens array 3. Each beam is then reflected from a mirror located on input movable mirror array 4 to another mirror on output mirror array 5. The light beams then travel through lens array 6 to output fiber array 7. Thus, a given beam is switched from an input fiber of input fiber array 2 to an appropriate output fiber of output fiber array 7 by being redirected by mirror arrays 4 and 5.
For this type of optical networking application, the intensity of the signals at the input or the output of the switch may be monitored to verify that the network is operating properly. Thus, a fiber tap array 9 is optically coupled to the fibers of input fiber array 2. The light beams traveling through each fiber of fiber array 2 are then sampled by diverting a portion of the beams through fiber tap array 9 to receivers in electrical receiver array 91. The receivers in receiver array 91 may convert the optical signals into digital electronic signals, or an optical switch may be used to multiplex the signals into a single electrical receiver. A disadvantage of this approach is that an individual tap fiber in tap array 9 must be connected to each input fiber of input array 2. Another disadvantage is that an individual receiver must be connected to each tap fiber. When using an electrical receiver array, or an additional optical switch has to be used when using a single electrical receiver. Therefore, the cost of monitoring the signals using this approach can be very high.
Another type of optical tap samples selected wavelengths or a selected range of wavelengths from one or more light beams to perform the monitoring function. This type of tap may include filters that pass the selected wavelength band. Other types of optical taps may include prisms or mirrors that direct a portion of the light beams in a separate direction that is not parallel to the path of the light beams that propagate through the switch along a signal path. Certain optical switch taps known in the art require additional components in the path of the light beams. Additional components introduce complications and variables relating to power loss and alignment difficulties, for example.
An all-optical switch provides very high data rates for data networks using optical signals in the form of light beams. In an all-optical switch, the light beams are not required to be converted into electrical signals for switching between input fibers and output fibers.
For an all-optical switch to operate properly, the mirror devices must be spatially arranged such that the waists of the light beams are at an optimum position relative to the surface of the mirror devices. Monitoring the light beams to determine if an all-optical switch is operating properly, however, can be problematic. For example, if an optical device is to be inserted into an optical path to monitor the light beams, the inserted device can increase the path length, which can cause insertion loss. Insertion loss relates to the loss of optical power within the optical switch. An increase in insertion loss can adversely affect the intensity of the light beams passing through the optical switch thereby causing the output power of the switch to fall below the requirements of the network or other optical system in which it is used.
For one embodiment, an all-optical switch includes an array of input optical fibers and an array of output optical fibers. A rhomboidal prism beam splitter is positioned in operative relationship with the array of input fibers such that when an array of optical beams is propagated from the array of input optical fibers, the beam splitter splits the array of optical beams into a signal path and a tap path. The rhomboidal prism beam splitter includes a partial internal reflection (PIR) surface and a total internal reflection (TIR) surface. The PIR surface and the TIR surface cooperate to split the array of optical beams into the signal path and the tap path such that the signal path is parallel to the tap path upon exiting the beam splitter.
Other features and advantages of the present invention will be apparent from the accompanying drawings and from the detailed description that follows below.